This application is based on an application No. 11-172072 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
This invention relates to a manufacturing method for a color cathode ray tube (hereafter abbreviated to CRT) and in particular to a manufacturing method for a glass substrate having a phosphor layer on its inner surface, that is used for a front panel of a CRT.
2. Description of Related Art
FIG. 1 is a perspective view of a conventional color CRT that has been partially cut away to show its interior. The color CRT shown in the drawing includes a glass envelope formed by joining together a front substrate 1, a funnel 91 and a neck 92, electron guns 93 that are inserted into the neck 92, a deflection yoke 95 that deflects electron beams 94 emitted by the electron guns 93, a phosphor layer 96 formed on the inner surface of the front substrate 1, color-selecting electrodes 97 positioned at fixed intervals on the side of the phosphor layer 96 nearer to the electron guns 93, and a magnetic shield 98. Here, the edges of the front substrate 1 are surrounded by a low barrier wall, and the term xe2x80x98inner surfacexe2x80x99 refers to the curved surface of the front substrate 1, but does not include the surface of the barrier wall.
FIG. 2 is an enlargement in cross-section of part of the inner surface of the front substrate 1, used to illustrate the structure of the phosphor layer 96. As shown in the drawing, a black film 99 with a thickness of around 1 micron is formed in stripes placed at fixed intervals on the inner surface of the front substrate 1. Then, colored phosphor stripes 3, 4 and 5 including phosphor particles with a diameter of 7 to 8 microns in the colors red, green and blue are formed in a specified positional relationship in the intervals between the stripes of black film 99. A reflective layer (not shown in the drawing) is placed on top of this structure, thereby forming the phosphor layer 96. The phosphor stripes 3, 4 and 5 emit light in their respective colors when the electron beams 94 strike the phosphor layer 96 via the color-selecting electrodes 97.
A conventional slurry method is used to form the phosphor layer 96. Such a method is described briefly below, with reference to FIGS. 3A and 3B.
A slurry 2 is formed from a photoresist in which phosphor particles have been suspended, the photoresist consisting of an aqueous solution of polyvinyl alcohol (PVA) to which an aqueous solution of ammonium dichromate (ADC) has been added. As shown in FIG. 3A, the front substrate 1 is positioned so that the inner surface faces upwards, and is tilted slightly at a fixed angle. Then the slurry 2 is poured onto the front substrate 1 while it is rotated slowly in the above-mentioned position, thereby gradually spreading the slurry 2 over the inner surface of the front substrate 1. The arrow in the drawing shows the direction in which rotation is performed. Once the slurry 2 has covered the entire inner surface of the front substrate 1, the front substrate 1 is tilted to the position shown in FIG. 3B and then rotated at high speed, spinning off excess slurry, and thereby forming a phosphor film of an even thickness. This phosphor film is then dried using a heater or warm air. Next, the color-selecting electrodes 97 are fixed at a certain distance from the inner surface of the front substrate 1 and exposed, before being developed using warm water or similar to form a phosphor stripe pattern in a specified color. This process is repeated in turn for each of the green, blue and red phosphors, thereby forming a phosphor pattern having the three specified colors. Following this, an organic film and then an aluminum evaporation film are formed on top of the structure, completing the formation of the phosphor layer 96 for the color CRT.
One important point to consider when using a slurry method to form the phosphor layer 96 is the need to achieve a layer of a uniform thickness when using the spinning process. An uneven phosphor layer will cause disparities in the amount of light emitted by the phosphor layer, thereby generating irregularities of light and shade on the screen surface. Furthermore, if the thicknesses of the phosphor layers for the three phosphors green, blue and red on the front substrate 1 vary at different points on the front substrate 1, the luminance for each color will be different. As a result, the brightness of the three colors will vary from place to place on the substrate 1 and white uniformity will be markedly reduced. One method for improving this situation and increasing white uniformity is described, for example, in Japanese Laid Open Patents Nos. 59-186230 and 6-203752. These documents disclose a technique for achieving an even phosphor layer by a combination of spinning the front substrate 1 with its inner surface facing upwards, and spinning the front substrate 1 with its inner surface facing downwards, once slurry 2 has been poured and spread over the surface of the front substrate 1.
However, the above-described related art technique makes it more difficult to recycle or reuse the excess slurry, and so the method illustrated in FIG. 3A and 3B is generally used to drain off excess slurry, and achieve an even phosphor layer. If this method is used, excess slurry can be recycled using simple recycling equipment, and there is little deterioration in the quality of the recycled slurry.
When the front substrate 1 is positioned horizontally with its inner surface facing upwards, the tilt angle is said to be 0xc2x0. Thus, a greater amount of excess slurry will be drained off if a larger tilt angle is used in the draining process. At the same time, however, the phosphor particles deposited on the inner surface of the front substrate 1 are loosened by the force of gravity and so are more likely to drop off. This reduces the amount of friction between the inner surface of the front substrate 1 and the phosphor particles, and accordingly reduces the concentration of phosphor particles on the front substrate 1 when high-speed rotation is performed in the spinning process, as described above.
Furthermore, the centrifugal force generated during the high-speed rotation performed in the spinning process may have a detrimental effect, particularly during the formation of the phosphor pattern for the second and third colors. This effect occurs if the orientation of the centrifugal force generated on these occasions has a certain relationship with the orientation of the grooves created by the phosphor pattern(s) of the colors that have already been applied. When phosphors are applied in a stripe pattern, this occurs when the orientation of the centrifugal force is parallel with the orientation of the stripes, in other words an orientation moving out from the center of the front substrate 1 towards its top and bottom edges (FIG. 4). Alternatively, when phosphors are applied in dot triads, this occurs when the centrifugal force has an orientation moving out diagonally towards the four corners of the front substrate 1. In either of these cases, phosphors are forced out from the central part of the front substrate 1 towards its edges, so that the concentration of phosphor particles in the central part of the front substrate 1 is reduced.
If the front substrate 1 has a large curvature radius, that is if it is virtually flat, the above tendencies are more marked, since the friction between the phosphor particles and the inner surface is reduced, making the movement of phosphor particles from the center of the inner surface toward its edges more likely. Conversely, if the tilt angle of the front substrate 1 in the draining and spinning processes is small, excess slurry which could not be removed is likely to accumulate on the barrier wall surfaces of the front substrate 1. This also causes irregularities in the concentration of phosphor particles to be generated on the inner surface of front substrate 1 during the spinning process. Note that such irregularities in the concentration of the phosphor particles are avoided when the pattern for the first color phosphor is formed, since application of a previous phosphor pattern has not created grooves on the inner surface of the front substrate 1.
In order to overcome the above problems, the object of the present invention is to provide a means of removing a sufficient amount of excess slurry, while restricting irregularities in the concentration of phosphor particles to form a phosphor pattern having an even thickness.
The above object is realized by a method for manufacturing a glass substrate on one surface of which a phosphor layer has been formed. The glass substrate is used for a front panel of a color cathode ray tube. The manufacturing method includes the following. First, in an application process, a phosphor slurry in one color is applied onto an inner surface of a glass substrate on which a phosphor pattern in at least one color has already been formed. Then, in a spreading process, the glass substrate is rotated about an axis located at the approximate center of the inner surface to make the phosphor slurry spread out over the inner surface of the glass substrate. Following this, in a draining process, the glass substrate is tilted to a first tilt angle of more than 90xc2x0 to drain excess slurry off the inner surface of the glass substrate, a tilt angle being formed between a vertical axis and an axis orthogonal to an outer surface of the glass substrate. Finally, in a spinning process, the glass substrate is returned to a second tilt angle smaller than the first tilt angle, and rotated.
In this method, the tilt angle for the spinning process is smaller than that for the draining process, enabling a sufficient amount of excess slurry to be removed during the draining process, while restricting irregularities generated in the concentration of phosphor particles during the spinning process.
The most suitable setting for the tilt angle may be expected to vary according to variations in the shape of the glass substrate, materials, and composition of the phosphor slurry, but generally the most desirable setting for the draining processing is a tilt angle that does not exceed 130xc2x0. The reason for this is that too large a tilt angle for the draining process will make it easier for phosphor particles to be loosened, thereby causing irregularities in the concentration of phosphor particles in the spinning process. Meanwhile, it is also desirable that the tilt angle in the draining process be no less than 105xc2x0. This enables efficient slurry to be drained off.
Furthermore, it is desirable that the glass substrate is rotated in the draining process at a rotation speed slower than the rotation speed used in the spinning process. This prevents excess slurry from remaining in places on the surface of the glass substrate.
For more efficient draining of excess slurry, the tilt angle for the spinning process should exceed 90xc2x0. However, this limitation need not apply.
It is further desirable that the tilt angle in the spinning process is no more than 130xc2x0. If an angle exceeding 130xc2x0 is used, irregularities in the concentration of phosphor particles are likely to be generated by phosphor particles being loosened from the surface of the glass substrate.
If the curvature radius of the glass substrate is approximately 10000 mm than the tilt angle should be no more than 110xc2x0 in the spinning process. A larger curvature radius will make the movement of phosphor particles from the center toward the edges of the inner surface more likely when the glass substrate is rotated at high speed. In this case it is necessary to preserve friction between the inner surface and the phosphor particles, so the tilt angle for the spinning process should be reduced.
Furthermore, the difference between the tilt angles in the draining and spinning processes should be no less than 5xc2x0 and no more than 20xc2x0. If the difference between the tilt angles for the draining and spinning processes is too large, excess slurry adhering to the barrier walls will splatter onto the inner surface when the tilt angle is returned while the high-speed rotation of the spinning process is being performed, thereby generating irregularities in the concentration of phosphor particles.
In the application process, the phosphor slurry is applied to the approximate center of the inner surface of the glass substrate. This ensures that phosphor particles are deposited on the center of the inner surface. Furthermore, in the spreading process, the glass substrate is tilted at a specified tilt angle when rotation is performed. This ensures that phosphor: particles are applied to the entire inner surface. In this case, the tilt angle should be less than 90xc2x0.
The object of the present invention is achieved by a color cathode ray tube manufacturing method including the following. In an application process, a phosphor slurry of one color is applied onto an inner surface of a glass substrate on which a phosphor pattern in at least one color has already been formed, the glass substrate being for use as the front panel of a cathode ray tube. Then, in a spreading process the glass substrate is rotated about an axis located at the approximate center of the inner surface to make the phosphor slurry spread out over the inner surface of the glass substrate. Following this, in a draining process the glass substrate is tilted to a specified tilt angle of more than 90xc2x0 to drain excess slurry off the inner surface of the glass substrate, the tilt angle being formed between a vertical axis and an axis orthogonal to an outer surface of the glass substrate. Then, in a spinning process, the glass substrate is returned to a tilt angle smaller than the specified tilt angle for the draining process, and rotated at a specified rotation speed. Finally, in a cathode ray tube assembly process, after a phosphor layer including phosphors in a specified plurality of colors has been formed on the glass substrate, the glass substrate is fitted together with other glass parts to assemble the cathode ray tube, and a near vacuum is formed in the cathode ray tube. Irregularities in the luminance of a color cathode ray tube manufactured using this method are restricted, and improved white uniformity achieved.